Previously known fireplace units which pull room air in under the fire enclosure bed are subject to the effects of radiation from the fire downwardly against the bed plate, recessitating the employment of a heavy construction which in turn adversely effects heat transfer. The thickness of the bed plate can be reduced somewhat by reason of reliance on the cooling effect of the underlying incoming room air. However, this cooling effect of incoming room air remains inefficient because of the relatively low velocity thereof and the uneven velocity pattern caused by any obstructions to air flow in the underlying passageway. This in turn, has a detrimental (in fact, dangerous) result that the bed plate becomes overheated in areas of low velocity or stagnation of air flow. Designers have resorted to extra thick bed plates in order to avoid burnouts, and thus have incurred additional cost in the construction of heavy units which increase shipping cost and render installation more difficult. Design has thus become a compromise between economy, light weight, and efficiency on one hand and safety and durability on the other.
It has also been recognized in the past that it is desireable to provide an ash transfer duct passing from the bed plate through the underlying room air passageway to a point of registry with the conventional ash chute of the fireplace hearth. However, in previously known structure, this duct completely isolates the bed plate in the area of the trap door for ashes from any heat exchange with the underlying room air and creates areas of low velocity or stagnation in the room air passageway surrounding the duct, thus increasing the likelihood of burn-out in that area.